1. Field of the Invention
The invention relates to a process for rolling a metal product and applies more particularly to hot rolling of flat products such as slabs or bands originated from a shaping mill or from continuous casting.
2. Description of Related Art
Hot rolling usually takes place in successive rolling stages in a unit comprising one or several roll stands. Each roll stand can be used as a reversible mill performing a number of reducing passes, alternately in one direction and the other, until the desired thickness is achieved. But, a single rolling pass can be carried out in each stand. The unit operates then as a tandem mill, whereas the rolled product is taken simultaneously in all the stands and its thickness is reduced successively in each roll stand.
The invention applies especially to hot rolling of steels and their alloys, but can also be used, in certain conditions, for rolling non-ferrous metals such as aluminum and its alloys.
Generally, a mill comprises a rigid holding stand with two separate roll standards between which are provided at least two working rolls, superimposed, thus forming a gap enabling the product to be rolled to run through the said gap. In a conventional, so-called quarto arrangement, the working rolls rest each on a back-up roll of larger diameter. In a so-called sexto assembly, idling rolls are interposed between the working rolls and the back-up rolls.
At least the back-up rolls are fitted, at their ends, with journals rotating inside chocks that are mounted to slide into windows provided respectively on both standards of the stand, parallel to a clamping plane, generally vertical, passing more or less through the axes of the working rolls.
The mill is associated with means to control the running of the product between the rolls, at a certain forward speed. In the case of a reversible mill, which rolls alternately into two opposite directions, the forward control means consist, generally, of two roller tables, respectively, one roller table placed upstream of the stand, in the running direction, in order to control the engagement of the product and another roller table, placed downstream, in order to receive the product upon completion of the rolling operation.
In hot rolling, the product is heated, before rolling, up to a temperature of approx. 1200xc2x0 C. in the case of steel, in order to facilitate deformation of the metal and its flow between the rolls. Generally, indeed, in a rolling process, the product exhibits at the inlet of the roll stand a thickness greater than the distance between the rolls and, when it contacts the said rolls, it is driven by a friction effect, then pinched between both rolls, whereas the metal continues flowing and being reduced in thickness, until a thickness more or less equal to the distance between the generatrices opposite both working rolls is achieved. Thus, a roll nip can be defined, delineated by the arcs of contact between each roll and the product.
Rolling, therefore, starts with a raw part such as a slab or a band of variable thickness, which may range between a few millimeters and several hundred millimeters and to each pass corresponds a reduction in thickness that may vary, for instance, from 50 mm to a few ten millimeters.
During rolling, the rolls tend to move away from one another and must therefore be held in place by an opposite roll load which, in a quarto mill, is applied to the chocks of the back-up rolls.
These clamping means are thus used, on the one hand, for prior adjustment of the distance between the rolls and, on the other hand, for maintaining the said distance during the roll pass. They generally consist of screws or hydraulic jacks mounted on the roll stand and resting respectively on both chocks of a back-up roll, whereas the other is blocked in its upward motion. However, other arrangements are possible. For instance, back-up rolls can be used, which comprise a shell mounted to rotate round a fixed shaft and resting on the said shaft via a series of jacks. These jacks then constitute clamping means exerting the rolling load, which is thus distributed over the whole length of the gap.
In all cases, under the effect of the rolling load, certain members of the roll stand will inevitably yield to some extent, thereby increasing slightly the distance between the rolls, which had been adjusted without any product, and therefore causes the crushing foreseen to be reduced. To realise accurately the requested reduction in thickness, the yielding value must be assessed, so that it can be compensated for as exactly as possible.
The rolling load to be applied for keeping a given distance between the rolls depends on how the product will be deformed in the nip between the rolls.
Conversely, the maximum reduction possible in thickness depends on the rolling load that can be applied, taking into account the capacities of the mill.
The reduction in thickness that can be achieved at each pass is therefore limited and this is why a raw product is rolled, normally, in several successive passes, each determining an elementary reduction in thickness, compatible with the capacity of the mill. The total reduction in thickness from a raw thickness eo down a final thickness en can be achieved in n passes according to a progressive thickness reduction process, called a rolling scheme, which depends on the capacity of the mill and on the adjustment means available, on the mechanical and physical features of the roll stand and of the product, as well as the thickness and evenness tolerances to be adhered to.
According to the capacities of the unit available, a single rolling scheme can be defined in which, at each pass, the same average reduction in thickness is achieved. The number of passes to be carried depends then, simply, on the total reduction in thickness to be provided.
However, it may prove necessary to increase the number of passes since the selected average reduction in thickness must be determined so as to remain compatible, for all the passes, with the characteristics of the product and of the roll stand. Still, to improve the productivity, it obviously pays to reduce, as far as possible, the number of passes to be carried out.
But it has also appeared that the final quality of the product, and especially its evenness, depended on the conditions in which rolling is performed and that all the thickness reduction schemes are not equivalent when a product of a set quality should be achieved.
For example, even if a certain temperature of the product can be defined at the beginning of the rolling process, this temperature varies from one pass to the next. Indeed, the product cools down during the waiting time between two successive passes, but the deformation of metal causes, conversely, the product to be heated during the pass and it may prove necessary to cool the product down between two passes in order to prevent excessive cumulated heating.
Still, the deformation conditions of the product, which determine the rolling load to be exerted, depend obviously on the nature of the metal and its temperature.
It therefore seems judicious, in order to obtain a product with set qualities, to adhere to an optimum pattern that depends not only on the mechanical capacity of the installation, but also on the final quality requested for the product.
The last few years have witnessed attempts to automate the rolling process of a flat product enabling to achieve the thickness foreseen with good evenness and by using a minimum number of passes without overloading the roll stand(s).
In such a system, it is necessary to control, at each pass, the adjustment of the clamping means in order to apply between the working rolls, a rolling load enabling to realise the maximum reduction in thickness compatible with the capacity of the mill. This rolling load is assessed in relation to the different rolling parameters on which depend the running conditions of the metal in the chock, notably the reduction in thickness to be provided, the forward speed and the temperature of the product when entering the mill.
According to the practice known until now in the most sophisticated units, on the basis of global parameters such as, for instance the flow stress of a metal in relation to its grade and to its temperature, table of references of its rolling conditions, observed previously for a known steel, can be drawn in order to deduce the conditions to be adhered to when the same steel lies again in the production programme of a unit.
To this end, the predictable rolling load should be assessed in each case. However, this load can only be appraised globally on the basis of the observations made during previous rolling operations. Such an estimate is not accurate enough to adjust the rolling conditions during each pass so as to obtain effectively the optimum reduction in thickness and, in particular, compensate for the yielding effect.
The invention remedies this shortcoming and suggests, thanks to improved modelling technology, a new process enabling to determine with greater accuracy the rolling load to be applied in order to follow a rolling scheme. Moreover, the invention enables to act automatically and in real time on the settings of the mill in order to modify the said settings at each pass in relation to the measurements taken during the previous pass, so that the rolling scheme can be adapted permanently while optimising the settings at each pass.
The invention therefore generally relates to a rolling process of a metal product in a unit comprising:
a holding roll stand with two separate roll standards,
at least two working rolls, superimposed between the standards of the roll stand,
means to control the forward motion of the product during the rolling operation in a nip delineated by two arcs of contact of the product with both rolls, between an inlet section and an outlet section of the nip,
clamping means resting, respectively, on the rolls and on the roll stand, for adjusting the distance between the working rolls corresponding to a reduction in thickness to be carried out and for maintaining the said distance during the roll pass, by applying, between the working rolls, a rolling load that depends on the mechanical and physical characteristics of the roll stand and of the product and on the flow conditions of the metal in the roll nip, and determines a yield effect of the various members of the stand tending to increase the said distance e,
means for adjusting the said clamping means, controlled by a computer associated with a mathematical model.
According to the invention, the computer associated with the mathematical model determines before each pass x, a foreseeable value of the flow stress of the metal corresponding to the deformation to realise in the pass x considered, while taking into account the evolution, during the rolling operation, of the microcrystalline structure of the metal making up the product to be rolled, and the rolling load Fx to be applied in order to achieve the requested reduction in thickness, is calculated before each pass x according to the value thus predicted for the flow stress and the evolution of the said stress during the rolling operation.
Particularly advantageously, the rolling load Fx to be applied for a rolling pass is calculated while taking into account the predictable variation, along the nip, of the flow stress of the metal during the said pass x.
To this end, the rolling nip is divided into a series of p adjacent elementary portions M1, M2, . . . Mi, . . . Mp, each corresponding to an elementary length of forward travel of the product between the rolls, with an elementary deformation xcex5i of the product in each portion M1 between an inlet section of thickness ei-1 and an outlet section of thickness ei, whereas, on the basis of data provided by the mathematical model, the computer determines, for each portion Mi, a predictable value "sgr"I of the flow stress of the metal, corresponding to the said elementary deformation xcex5i and deduces therefrom the elementary rolling load dFi to be applied to the considered portion Mi in order to provide the said elementary deformation xcex5i and that, by integrating the elementary loads dFi into the successive portions M1, M2, . . . Mi, . . . Mp, the computer determines the global rolling load to be applied in order to achieve the requested reduction in thickness and controls, in relation to the global load thus calculated, the adjustment of the clamping means for maintaining the distance between the rolls, in order to achieve the requested reduction in thickness (exxe2x88x921xe2x88x92ex), while taking into account the yield conditions of the metal along the nip and the yield effect resulting from the said global load.
It should be noted, however, that the invention enables thus to determine the rolling load Fx to be applied during a pass x while taking into account the predictable value of the flow stress of the metal resulting from the evolution of the microcrystalline state of the metal during the previous passes.
Generally, the rolling operation is performed according to a rolling scheme enabling to achieve in n successive passes a global reduction in thickness (exxe2x88x921xe2x88x92ex).
According to another characteristic of the invention, the computer determines, by iteration, the rolling scheme to be adhered to while computing beforehand, for each pass, x, the maximum reduction in thickness leading to a predictable rolling load Fx compatible with the capacity of the mill, in relation to a number of rolling parameters comprising the thickness and the temperature of the product as well as its forward speed before entering the said pass x, in order to take into consideration the predictable evolution of the microstructure of the metal from one pass to the next.
In particular, the computer can be associated with permanent measuring means, during the pass, of the effective values of a set of rolling parameters comprising the rolling load applied at each moment, the forward speed of the product and the temperature of the said product respectively at the inlet and the outlet of the mill. Thus, at each pass x, the computer can compare these effective measured values with the values of the said parameters taken into account initially for the said pass x in the determination of the rolling scheme, in order to review the calculation of the said scheme and to add, if needed, correction factors to the parameters taken into account, in order to adapt the rolling scheme in the following passes.
According to a preferred embodiment of the invention, in order to take into account the microcrystalline evolution of the metal during the rolling operation, at least one modelling equation valid for a family of metals having an analogue microcrystalline behavior is established, on the basis of hot deformation tests carried out on sample pieces of at least one typical metal of this family, whereas the said equations depend on a set of parameters associated with the composition of the typical metal. The initial equations thus established are grafted to the mathematical model and, for rolling a product consisting of a metal of the same family as the typical metal, the model is calibrated for the metal to be rolled while modifying the parameters of the said theoretical equations in relation to results of deformation tests performed on a metal whose composition is at least similar to that of the metal to be rolled.
Particularly advantageously, to define modelling equations, an intermediate value can be determined, associated with the deformation speed of the metal and varying in a more or less linear fashion in relation to the flow stress in at least one deformation domain and, on the basis of deformation tests realised for a series of deformation temperatures and speeds held constant, a work-hardening diagram is established for which the variations of the said intermediate value can be represented approximately, in the said domain of deformation, by a family of straight lines to which corresponds at least one linear differential equation, associating the deformation with the flow stress and liable to be integrated by the computer.
On the basis of such a work-hardening diagram, we can establish at least two differential equations associating the deformation with the flow stress, respectively a first equation, linear in shape, giving by analytical integration, an expression of deformation in relation to the flow stress and a second equation liable to be integrated digitally in order to determine the predictable flow stress corresponding to a deformation to achieve.
According to another embodiment, whereas the modelling equations have been established initially for a typical metal and grafted to the mathematical model, these equations can be calibrated for the metal to be rolled, while performing first of all at least one rolling pass, at least one product made up of the metal to be rolled, in at least one roll stand adjusted conventionally and in measuring, during each pass, on the one hand the rolling load actually exerted and, on the other hand, the rolling parameters used by the computer in order to determine, using initial modelling equations, the rolling load to be exerted theoretically. Using a digital regression method, the modifications to be made to the parameters of the said initial equations can be defined in order to provide modelling equations specific to the metal to be rolled.
The invention also covers a particularly advantageous method to exploit the test results in order to establish the modelling equations. In such a method, on the basis of deformation test results each carried out at constant temperature and at constant deformation speed:
a first work-hardening diagram is established, comprising a series of representative curves, for each temperature T, of the variation of the temper-rolling rate xcex8=d"sgr"/dxcex5 in relation to the flow stress "sgr",
the digital data relating to each curve is transformed to establish a second normalised work-hardening diagram comprising a series of curves representative of the variation, in relation to the normalised running stress "sgr"*="sgr"/xcexc(T), whereas xcexc(T) is the shear modulus at the temperature considered,
whereas the said curves have each at least a portion more or less rectilinear situated in at least one domain II, III of the diagram, and the said rectilinear portions are more or less parallel in each domain,
each more or less rectilinear portion is modelled according to a first equation of the type:
k"sgr"*+kxe2x80x2=2xcex8*"sgr"*=b2dxcfx81/dxcex5
xe2x80x83while using as an intermediate variable, the dislocation density xcfx81 such that
"sgr"=xcexcb
xe2x80x83and an analytical integration of the first equation is performed in order to establish, at least for each of the domains II, III, a second modelling equation
xcex5=xe2x88x922/kb2[Xsln(1xe2x88x92x/xs)+X]+xcex
xe2x80x83by defining x=b="sgr"/xcexc="sgr"* and xs=xe2x88x92kxe2x80x2/k, whereas xcex is an integration constant,
whereby the parameters k and kxe2x80x2 are determined, for each of the domains II, III, on the basis of the rectilinear portion of a curve of the second work-hardening diagram corresponding more or less to the predictable temperature of the metal and to the predictable deformation speed when entering the roll stand.
In each of the domains II, III of the work-hardening diagram, the coefficients k and kxe2x80x2 of the first modelling equation can be determined by the computer while following a digital regression method, on the basis of the temperature and of the parameters representative of the crystalline state of the metal when entering the roll stand.
In order to take into account the evolution of the flow stress along the rolling nip, the former is divided into a series of successive portions M1, M2, . . . , Mi, . . . Mp, each corresponding to an elementary deformation xcex5i, and the computer determines before each pass, in relation to the roll parameters measured at the inlet of the stand, the predictable flow stress "sgr"i in each of the said portions Mi by digital integration reverse of the second modelling equation in relation to the elementary deformation xcex5i to realise in the considered portion Mi and deduces therefrom the elementary rolling load dFi to be applied in the said portion Mi, whereas the global rolling load is calculated by integration of the said elementary loads along the nip.
The invention also covers numerous other advantageous characteristics that are subject to the sub-claims.
It should be noted that, owing to the fact that it enables to calculate accurately, and at any moment during the rolling operation, the predictable value of the flow stress, the process according to the invention could be integrated to several levels in the rolling process.
In particular, as the rolling parameters are measured during each roll pass, the computer may check whether the global rolling load calculated in relation to the reduction in thickness predicted by the rolling scheme is compatible with the capacities of the unit and whether the said predicted reduction in thickness makes optimum use of the said capacities and it can also check the said capacities and modify, if needed, the rolling scheme for the following passes.
But the invention will be understood better using the following description of a peculiar embodiment, given for exemplification purposes and illustrated by the appended drawings.